Our long-term goal is to understand how the control of protein synthesis in skeletal muscle can be manipulated for preventive and therapeutic purposes. The objective here is to gain an understanding of how to prevent and/or reverse inactivity-induced loss of skeletal muscle mass. Inactivity encompasses a number of conditions that lead to loss of muscle mass including sedentary life styles, chronic bed rest, casting, limb suspension, and immobilization. Each of these inactivity-related conditions is thought to share a common feature, i.e. a delayed and reduced magnitude in the response of skeletal muscle protein synthesis to an anabolic stimulus, which has been referred to as anabolic resistance. Our central hypothesis is that dysregulation of the mammalian target of rapamycin complex 1 (mTORC1) and/or protein synthetic control mechanisms is the underlying cause of the anabolic resistance-induced loss of skeletal muscle mass. Our hypothesis has been formulated on the basis of our past accomplishments in defining the regulation of mTORC1 signaling and protein synthesis in skeletal muscle under a variety of physiological and pathophysiological conditions. To test our hypothesis, we will pursue an experimental protocol in which control rats and rats subjected to short-term unilateral hindlimb immobilization will be placed on a meal-feeding regimen designed to elicit the anabolic resistance of protein synthesis in skeletal muscle. We will then perform a detailed analysis of components of the mTORC1 signaling pathway, including novel upstream inputs such as p53 and Sestrin 1, 2, and 3, as well as selected novel regulatory mechanisms that mediate control of protein synthesis, including truncated 4E-BP1 and PDCD4. We will also define the regulation and function of two previously identified potential therapeutic targets for manipulating skeletal muscle protein synthesis, i.e. the mTORC1 repressor, regulated in development and DNA damage responses (REDD1) and the growth regulatory protein, eukaryotic initiation factor (eIF2B)?. The proposed studies encompass the following three specific aims: (1) assess responses of the mTORC1 signaling pathway and selected regulatory mechanisms that mediate control of protein synthesis under conditions that elicit anabolic resistance in skeletal muscle; (2) delineate the mechanism of action of REDD1 and define its potential as a therapeutic target for the regulation of mTORC1; and (3) delineate the molecular mechanisms that mediate degradation of eIF2B? and continue to evaluate its therapeutic potential as a regulator of skeletal muscle mass. Overall, we expect the proposed research to reveal molecular targets that can likely be manipulated pharmacologically resulting in new and innovative approaches to the prevention and treatment of muscle wasting conditions.